Strain measuring instrument



March 6, 195] w, so wm 2,544,400

STRAIN MEASURING INSTRUMENT Filed Jan. 19, 1944 3 Sheets-Sheet 1 AMPLIFIER :7: AAA AAA AAAA 4235 1 POWER :5 SU PLY TVIIVVVVYVVV AMPLIFIER I lz vvv WIN! (I I 27 |8 POWER r29 L JI SUPPLY VIIVAVAVAVAVA AvAvAvAvA/L mama g A TTOR NE Y March 6, 1951 w. A. SOURWINE STRAIN MEASURING INSTRUMENT 3 Sheets-Sheet 2 Filed Jan. 19, 1944 INVENTOR.

WILLIAM A. SOURWINE A vvvvvvvvvv ATTORNEY March 6, 1951 w, SQURWINE 2,544,400

STRAIN MEASURING INSTRUMENT Filed Jan. 19, 1944 5 Sheets-Sheet 5 FIG. 4

INVENTOR. WILLIAM A. SOURWINE ATTORNEY Patented Mar. 6, 19,51

STRAIN MEASURING INSTRUMENT William A. Sour-wine, Eggertsville, N. Y., assignor to Curtiss-Wright Corporation, a

of Delaware corporation Application January 19, 1944, Serial No. 519,076

3 Claims. (Cl. 73-885) 1 This invention relates to mechanism for electrically measuring and indicating quantities which can be caused to actuate slight variations in electrical resistance. It is particularly applicable to the measurement and indication of structural strains, such as in aircraft, ships, land vehicles, bridges, highways,. subways, towers,

pressure vessels and tanks, dams, cranes and hoists, machinery, and ordnance, by means of resistance type strain gages.

Commercial strain gages are composed of predetermined lengths of special resistance wire, and are attached to the material in which strain is to be measured. Imposed strain causes the resistance of the wire to change, and this change in resistance is measured by incorporating the gage as one arm of a Wheatstone bridge. A compression strain causes decrease in resistance, while a tension strain causes increase in resistance. In previous practice, it has been customary to balance the bridge before imposing a strain thereon. This method was rather cumbersome, in that it required two separate balancing operations, with calculations of resistance for each balanced condition, and calculation of strain from the change in resistance. The mechanism of the present invention eliminates the necessity for one of these balancing operations,, and also eliminates the calculations, the amount of strain being measured directly in micro-inches per inch on a galvanometer or other suitable current measuring device.

In the drawings, Figure 1 is a general wiring diagram of a 'simple embodiment of the invention,: in which the direct current output of the bridge 'is supplied by a direct current amplifier, and the resulting amplified current measured on-;a--ga1vanometer reading directly in micro inches per inch; Figure 2 is a general wiring diagram of another embodiment of the invention, in which the direct current output of the bridge is changed to alternating current, the latter then amplified by an alternating current amplifier and a power supply unit, and the resulting amplified current measured on a galvanometer reading directly in micro inches per inch; Figure ,3..is a wiring diagram of a form of alternatingcurrent amplifier for use in the circuit shown in Figure 2 together with D. C. to A. C. transformer means for the amplifier input and rectifier means connected to the amplifier output for operating an indicator; and Figure 4 is a wiring diagram of a form of power supply unit for use in the circuit shown in Figure 2.

Referring now to Figure 1, the strain gage of gage l2 to change.

H or other resistance to porated as one arm of a dicated generally at H. pensating or control gage I3 is incorporated as another arm of the bridge H. The third arm of the bridge is composed of a fixed resistance l4, while the fourth arm is composed of a fixed resistance IS with an adjusting circuit in parallel thereto, such adjusting circuit being composed of a fixed resistance i6 and a rheostat or variable resistance H in series therewith. A battery [8 supplies current to the bridge through the circuits l9 and 20. The current output of the bridge is fed through circuits 2| and 22 to the direct current amplifier, indicated generally at 23, and the amplified current read on the milliammeter indicated generally at 24. A 3-pole switch 25 brings the circuit into operation, and is composed of three contact arms 26, 21, and 28, which operate to close circuitslQ, 20, and 22 respectively. The power supply unit 29, furnishes current to the amplifier 23.

Any number of bridges similar to bridge i I may be used with a single battery-amplifier-power supply-milliammeter set-up. Each bridge would have a separate switch 25 to throw in the'battery l8 and one circuit 22 to the amplifier 23. The remaining circuits 2| for all the bridges would be grounded or permanently connected to a common terminal at the amplifier 23.

In the operation of any bridge, the bridge is first balanced at zero strain by manipulation of the rheostat I1, until the milliammeter 24 registers no current. A strain is then imposed upon the element under stress, causing the resistance The current is again read on the milliammeter, which may be calibrated, directly in micro inches per inch. When more than one strain gage is to be read, this is preferably done by first bringing all the bridges into balance at zero strain, imposing strains on all the gages, and then reading the strains on the successive bridges by successively closing the switches 25.

The embodiment of the invention shown in Figure 1, has the objection that the current output from the direct current amplifier 23 increases continuously, or shows a drift, during the warming up period. This obviously lowers the accuracy of the apparatus and also renders the process of reading strains somewhat more laborious, due to the necessity of constantly bringing the bridges into balance at zero strain be measured in incor-" Wheatstone bridge in- A temperature combefore each reading is taken. To overcome these 3 difilculties, the apparatus shown in Figure 8 is employed, in which the direct current output from the bridge is transformed into alternating current, and the latter than amplified and measured on a milliammeter as before.

Referring now to Figure 2, the strain gage 42 or other resistance to be measured in incorporated as one arm of a Wheatstone bridge indicated generally at 4|. A temperature compensating or control gage 43 is incorporated as another arm of the bridge 4|. The third arm of the bridge is composed of a fixed resistance 44, while the fourth arm is composed of a fixed resistance 45 with an adjusting circuit in parallel thereto, such adjusting circuit being composed of a fixed resistance 46 and a rheostat or variable resistance 41 in series therewith. A battery 48 supplies current to the bridge through thecircults 48 and 50. The current output of the bridge is fed through grounded circuit 5| to the midpoint of a coil 62 and through circuit52 and vibratory reed interrupter II to the opposite ends of coil 62. Coil 62 is the primary of a transformer indicated generally at 6|. 63 of transformer 6| is connected to amplifier 53. A power supply unit, indicated generally at 59, furnishes power to the amplifier 53, and the.

output from the latter is registered on milliammeter 54, reading directly in micro-inches per inch. A 3-pole switch 55 brings the circuit into operation, and is composed of three contact arms I The secondary 4 for the tubes and serve to provide a low impedance path for the alternating current.

The plate supply voltage of plate 85, brought from a source of direct current through a circuit I02 (the positive terminal is indicated at I is loaded with two series resistances 86 and 81. The negative terminal IOI of the source of direct current is connected to the ground circuit A condenser 88 is connected to the circuit 88 (connecting resistances 88 and 81) and the ground circuit 5 I thus serving to reduce to a minimum the alternating component of the plate supply voltage of the plate 85. Likewise the plate supply voltage of plate 88. brought from the same source of direct current through circuit I02, is loaded withtwo series resistances I06 and I01. A condenser I08 is connected to the circuit I09 (connecting resistances I86 and I01) and the ground circuit 5|, thus serving to reduce to a minimum the alternating component of the plate supply voltage of the plate 88.

From well-known principles of vacuum tube operation it is seen that the current through the secondary 68 of the transformer 6| controls the grid supply voltage of the grid 83. The latter in turn controls the plate current of plate 85, the

56; 61, and 58, which operate to close circuits 48, 50, and 52 respectively.

A plurality of bridges may be incorporated with one battery-amplifier-power supply milliammeter set-up, as described in connection with Figure 1.

Referring now to Figure 3, the vibratory reed interrupter II is composed of a vibrating metallic reed I2 having a natural vibration frequency equal to the desired frequency of the current to be produced. This reed is directly connected to circuit 52. When it is set into vibration it alternately makes contact with contact arms I3 and 14 which are connected to opposite ends of the primary coil 62. The current thus passes through primary coil 62 in alternate directions and thus induces an alternating current in secondary coil 63. The reed I2 is started and kept in continuous vibration by means of a solenoid I6 supplied with alternating current through conductors I4 and I5 and terminals I and "I, this circuit being of the same frequency as the natural frequency of the reed 12.

The transformer 6| contains a core 64 which is grounded to the circuit 5|. One end of the secondary coil 63 is also grounded to circuit 5|, the opposite end being connected to one grid 83 of a twin triode vacuum tube 8|. The other grid 88 of the tube 8| is connected to the ground circuit 5| through two resistances 88 and 90. In parallel with resistance 80 there may be interposed another resistance or resistances, as hereinafter described, for the purpose of varying the degree of amplification to be obtained. The two cathodes 84 and 81 of the tube 8| are associated with the grids 83 and 86 respectively, and with the two plates 85 and 88 respectively. Grid 83 thus controls the plate current in plate 85, and grid 88 the plate current in plate 88. The heater 82 of the tube 8| is connected across the alternat current circuits I4 and 15. The two cathodes-84 and 81 are connected to the ground circuit 5| through the resistances 8| and 82 respectively in parallel with condensers 83 and 84 respectively.

These resistances provide the necessary grid bias alternating component of which passes through a condenser- I03, a resistance I04 and resistance 88, and thus controls the grid voltage of grid 86. Finally; the latter controls the plate supply voltage of the-plate 88, the alternating component of which passes through a condenser H0 and thus controls the grid supply voltage of the control grid I22 of a vacuum tube pentode I2I, as hereinafterdescribed. A resistance III connects this control grid circuit to the ground circuit 5|.

In the pentode I2 I, the cathode I23 is connected to the ground circuit 5| through a resistance I24 and a by-passing condenser I25. The heater I26 is connected across circuits I4 and I5. The suppressor grid I2! is connected to the cathode I22 in well-known fashion, while the screen grid I28 is connected to the positive input terminal I00 through the conductor I02, and to one end of the primary winding I32 of an iron-core transformer I3I. The opposite end of the primary winding is connected to the plate I29 of the pentode I2I. The secondary winding I33 of the transformer |3| is connected to the input terminals of a selenium rectifier I34, with a shunt resistance |4| connected across such input terminals. The positive output terminal of the rectifier I 34 is connected to the ground circuit 5|, the latter in turn being connected to one terminal I31 of the milliammeter 54. The negative output terminal of the rectifier I34 is connected to the other terminal I35 of the milliammeter 54. Across the terminals of the galvanometer there is connected a battery I 38 and a variable resistance I38 in series with each other. The purpose of this battery and resistance combination is to provide a means for balancing the amplifier output to secure a zero reading on the galvanometer at zero strain of the specimen under test. One deck I44 of a switch I 43 i also located in the battery circuit. The other deck I45 of the switch I43 is contained in the circuit 15.

By reference to the above description and drawings of the circuits of Figure 3, it will be seen that the apparatus therein depicted is a 3-stage alternating current amplifier, with resistance-capacitance coupling between the first two stages and transformer output coupling from the third stage. The degree of amplification obtained (according to the magnitude of the imposed strain) can be adjusted by means of the range control apparatus Indicated generally at I50. This operates to introduce a by-passing resistance around the resistance 90, thereby decreasing the signal voltage applied to the grid 86. The range control apparatus consists basically of two contact arms I5I and I52 respectively which are mechanically but not electrically connected together and thus operated in unison. In the position shown in Figure 3, contact arm I5I is connected to terminal I53, which is electrically isolated. Thus there is no resistance shunting or by-passing resistance 90. When the arm I5I is moved to the right, it comes into contact with terminal I54, thus bringing into the circuit a resistance I55 which is in parallel with resistance 90. When the arm I5I is moved still farther to the right, it comes into contact with terminal I56, thus bringing into the circuit a resistance I51 which is in parallel with resistance 90.

Contact arm I52 also has three positions, in which it contacts terminals I58. I59, and I60 respectively, corresponding to similar positions for contact arm I5I. Contact with these respective terminals brings into operation signal lights I63, I64, and I65 respectively. Thus when resistance 90 alone is in circuit, signal light I63 is in operation; when resistance I55 is in parallel with resistance 90, signal light I54 is in operation; and when resistance I51 is in parallel with resistance 90, signal light I65 is in operation.

The dotted lines in Figure 3 (except for the dotted lines connecting contact arms I5I and I52 and also the two decks of switch I43) show the position of shields for protecting certain circuits from the effects of stray currents.

The operation of the Wheatstone bridge and amplifier apparatus, shown in Figures 2 and 3 may be described as follows. The strain gage 42, together with the compensating arm 43, is incorporated in one of the bridges 4I, and the rheostat 41 adjusted so that the bridge is in as closely balanced a condition as it is possible to predict in advance. With the switch I5I in the position shown in Figure 3 and the entire resistance I39 thrown into the circuit and switch I43 closed, the switch 55 corresponding to the particular bridge in use is cautiously closed. Since the alternating current amplifier is unable to distinguish between a positive and a negative bridge output potential, the bridge is put into unbalance by adjusting the resistance 41 until the galvanometer deflection of galvanometer 54 is a preselected amount. With this condition, an increase of resistance in the arm 42 (corresponding to a tension load) will cause the galvanometer needle to deflect in one direction, while a decrease of resistance in the arm 42 (corresponding to a compression load) will cause the galvanometer needle to deflect in the opposite direction.

After the bridge is thus put into a preselected unbalance by adjustment of resistance 41,the amplifier output is balanced by adjusting resistance I39 until the galvanometer needle is at the zero point. A strain is then put on resistance 42, and the amount of strain read directly in microinches per inch on the galvanometer scale.

Referring now to Figure 4, this indicates generally the power supply unit which is used to supply power to the amplifier indicated in Figure 3. The power supply unit is supplied with a suitable source of direct current, such as a battery I80. The terminals for attachment of this source are indicated at I8I and I82, being the positive and negative terminals respectively. The negative terminal I82 is connected to the mid-point of the primary winding I9I of a transformer I90, through the grounded conductor I89. The positive terminals I8I is connected through switch I86, fuse I81, choke coil I88 and conductor I19 to a vibrating metallic reed I84 which alternately comes in contact with contact arms I83 and I88 respectively. These two contact arms are connected to opposite ends of the primary winding I9I. Thus it is seen that the vibration of reed I84 causes a direct current to pass alternately through each half of winding I9I. Condensers I94, I95, and I96 serve to absorb the current as contacts I83 and I85 are alternately opened and closed. The reed I84 is kept in continuous vibration by means of a solenoid I91 which is connected to the mid-point of the coil I9I and also to one end of the coil through condenser I95. Thus part of the current which passes through the lower half of coil I9I will pass through solenoid I91. This will occur, however, only during half a cycle, since current only passes through the lower half of coil I9I during half a cycle. Thus solenoid I91 will attract reed I84 during half a cycle only, and will keep it in its natural period of vibration.

The core I99 of the transformer I is grounded to the conductor I89. The transformer contains two secondary windings, one winding I92 furnishing alternating current for the amplifier of Figure 3, through the terminals I10 and HI. Winding I92 also furnishes current for the heater 2II of a vacuum tube rectifier 2I0. The other winding 203 of transformer I90 is connected at its opposite ends to the two plates 2I2 and MS of vacuum tube rectifier 2I0. Across the terminals of winding 203 is connected a condenser 204. The center point of winding 203 is grounded to the conductor I89.

Two condensers 206 and 201 from conductor I19, on the positive side of battery I80, to the grounded conductor I89, on the negative side of battery I80. These two condensers are connected to opposite sides of the choke coil I88.

The cathode 2I4 of rectifier tube 2I0 is connected in series with a choke coil 22I and a resistance 222 to the positive terminal I00 for the direct current output of the power supply unit. The negative terminal IOI for the direct current output is connected directly to the grounded conductor I89 and thence to the negative terminal I82 of battery I80. Fluctuations in the direct current output are smoothed out by means of condensers 2I6, 2I1, and 2I8.

The power supply unit is put into operation by closing switch I86.

The above discussion has been in connection with several specific forms of the invention. It is obvious, however,that many changes may be made in these forms without de arting from the spirit of the invention. Accordingly, it is understood that the invention is not to be limited except as defined by the appended claims.

I claim:

1. An apparatus for measuring strains as reflected by small variations in resistance of a strain gage whose resistance is variable in opposite directions from normal in response to strains applied to the gage in opposite directions, said apparatus comprising a Wheatstcne bridge having the input terminals thereof arranged for connection to a source of direct current whereby with the bridge balanced in the normal condition of the gage the bridge output current would vary are connected in polarity depending upon the direction of the strains subsequently applied to the gage, means for adjusting the resistance of another arm of the bridge for unbalancing the latter so that the bridge output current will be of the same polarity irrespective of the direction of the strains applied to the gage, means for transforming the bridge output current into an alternating current, the last-mentioned means comprising a transformer and a vibrating reed for reversing the polarity of the current applied to the transformer primary windings at a predetermined frequency, an alternating current amplifier for amplifying the alternating current from the transformer secondary windings, means for rectifying the output current from the amplifier, a galvanometer connected across the output of said rectifying means and biasing means shunting the galvanometer and adjustable to reduce the galvanometer current to zero in the said normal condition of the strain gage, said galvanometer measuring variations in either direction from zero and being calibrated to measure directly the value and direction of strains applied to the gage.

2. An apparatus for measuring strains as reflected by small variations in resistance of a strain gage whose resistance is variable in opposite directions from normal in response to strains applied to the gage in opposite directions, said apparatus comprising a Wheatstone bridge having the input terminals thereof arranged for connection to a source of direct current whereby with the bridge balanced in the normal condition of the gage the bridge output current would vary in polarity depending upon the direction of the strains subsequently applied to the gage, means for adjusting the resistance of another arm of the bridge for unbalancing the latter so that the bridge output current will be of the same polarity irrespective of the direction of the strains applied to the gage, means for transforming the bridge output current into an alternating current, an alternating current amplifier for amplifying the transformed bridge output current, means for rectifying the output current from the amplifier, a galvanometer connected across the output of said rectifying means and biasing means shunting the galvanometer and adjustable to reduce the galvanometer current to zero in the said normal condition of the strain gage, said galvanometer measuring variations in either direction from zero and being calibrated to measure directly the value and direction of strains applied to the gage.

3. An apparatus for measuring strains as reflected by small variations in resistance of a strain gage whose resistance is variable in opposite directions from normal in response to strains applied to the gage in opposite directions, said apparatus comprising a Wheatstone bridg having the input terminals thereof arranged for connection to a. source of direct current whereby with the bridge balanced in the normal condition of the gage the bridge output current would vary in polarity depending upon the direction of the strains subsequently applied to the gage, means for adjusting the resistance of another arm of the bridge for unbalancing the latter so that the bridge output current will be of the same polarity irrespective of the direction of the strains applied to the gage, means for transforming the bridge output current into an alternating current, an alternating current amplifier for amplifying the transformed bridge output current, means for rectifying the output current from the amplifier, an electrical indicating instrument connected across the output of said rectifying means and biasing means shunting said instrument and ad- Justable to reduce the instrument current to zero in the said normal condition of the strain gage, said indicating instrument measuring variations in either direction from zero for determining the value and direction of strains applied to the gage.

WILLIAM A. SOURWINE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,460,530 Brown et a1 July 3, 1923 1,665,397 Wunsch Apr. 10, 1928 1,939,067 Legg Dec. 12, 1933 1,961,965 Fisher June 5, 1934 2,077,833 Gieringer Apr. 20, 1937 2,114,298 Gunn Apr. 19, 1938 2,133,670 Schuchmann Oct, 18, 1938 2,229,009 Berry Jan. 14, 1941 2,329,841 Keinath Sept. 21, 1943 2,339,579 Milne et al. Jan. 18, 1944 2,389,615 Eder Nov. 27, 1945 

